A conventional circular secondary clarifier for treatment of wastewaters as depicted in FIG. 1, comprises an inlet (1) for conveying wastewater, a centre feed column (2) having openings (3) through which, wastewater is radially distributed in to the clarifier, an inlet well also referred to as baffle wall (4) is provided to prevent escape of solids along with the treated effluents, the clarifier bottom (5), having a slope preferably in the range 1 in 12 to 1 in 10, towards the center of clarifier, is provided to collect the settled solids referred to as sludge (9) to the center and the treated effluent (6) is collected into the peripheral channel called launder (7), the settled sludge (9) is collected at the center by a rotating scraper (8) and the sludge (9) is withdrawn under hydrostatic pressure from the bottom outlet (10) and collected in sump cum pump house (not shown) for either recycling to aeration tank or for sludge dewatering and thickening, before ultimate disposal. In the conventional clarifier, wastewater containing mixed liquor suspended solid (MLSS) enters the clarifier, through a high level inlet (3), which creates turbulence in the clarification zone and affects the settling of solids. Since the difference in the density of biological solids and wastewater is very less, the solids remain in suspension, and are carried away along with the outgoing wastewater, thereby deteriorating the treated effluent quality. This in turn affects the thickening of solids and reduces the underflow solids concentration.
The design of secondary clarifier is primarily based on solids loading rate (SLR) and surface overflow rate (SOR) usually expressed in kg/m2.d and m3/m2.d, respectively. The surface area of clarifier, for a given solids concentration (expressed in mg/l) and flow rate (expressed in m3/d) is governed by above-referred parameters (SLR, SOR), and the greater of the two is provided.
Reference may be made to Wastewater Engineering, Treatment and Reuse IV edition (2003), Metcalf & Eddy Inc., wherein the average SLR and SOR for clarifier following extended aeration unit, range from 1 to 5 kg/m2.hr (24 to 120 kg/m2.d) and 8 to 16 m3/m2.d, respectively, and the depth of clarifier varies from 3.5-6 m. Under these conditions, for a conventional clarifier, the surface area required for a given flow rate is more as compared to the improved clarifier, described in the present invention. Further, the recommended depth of conventional clarifier is also on the higher side, as compared to the improved clarifier described in the present invention. In other words, the design parameters for a clarifier, viz. SLR and SOR, can be reset to higher ranges to obtain a compact clarifier, which is capable of providing improved solids-liquid separation. However, in the conventional clarifiers it is not feasible to provide higher ranges for design parameters (SLR, SOR) due to the problem of effective solids-liquid separation.
Reference may be made to “Clarifier Design Manual of Practice, FD-8, WPCF, (1985, 2005)”, wherein the various conventional circular secondary clarifiers comprising center and peripheral feed arrangements with flow patterns are shown. In the clarifiers with central feed arrangement, the diameter of feed well is about 20-25% of the diameter of the clarifier, which in turn reduces the surface area for clarification. Therefore to account for diameter of feed well, the diameter of clarifier is increased by 20-25%, which increases the capital cost of the unit, whereas in the improved clarifier described in the present invention, the surface area required is less, as no feed well is provided. Conventionally, the solids enter the clarifier through a high level inlet (3), get dispersed immediately and mostly settle as individual solid. Some times, therefore to increase the floc size, a flocculation mechanism is also provided (not shown). This in turn increases the size and cost of the clarifier, whereas that described in the present invention, the floc size increases due to natural flocculation, and solids settle as a zone of sludge layer uniformly.
In the conventional peripheral feed type of clarifiers, the wastewater is fed through inlet ports all along the perimeter or through spiral roll pattern by introducing the flow into the clarifier near the bottom, wherein the flow pattern induced may carry away the solids accumulated at the bottom along with the incoming wastewater, thereby resulting in higher SS concentrations in the treated effluent. Further, the peripheral feed clarifiers with open ports or orifices have inherent problem of unequal distribution as flow changes, whereas in the improved clarifier described in the present invention, the flow is uniformly distributed, ensuring uniform thickening of sludge layer at the bottom of the clarifier.
Reference may be made to U.S. Pat. No. 4,005,019 (1977) wherein the disclosure relates to rectangular clarifiers for use in the gravity separation of suspended solids from liquids. The invention basically relates to the apparatus used in the gravity separation of suspended solids from liquids, comprising a gantry supported for movement above the liquid surface; gantry drive to cause the gantry to perform repeated passes over the said liquid surface; a sludge conveyor device to move sludge which has settled at the bottom of the clarifier. The gravitational separator described in the above-referred US patent clearly indicates that the invention is related to sludge removal mechanism for rectangular clarifiers and does not include measures like hydraulic energy dissipation and improved geometrical configuration to improve solid-liquid separation described in the present invention.
Reference may be made to U.S. Pat. No. 4,761,239 (1988), wherein the process for wastewater purification is disclosed, which is particularly suitable for raw wastewater clarification by reducing BOD and SS concentrations employing sequential addition of chemicals, viz. salts of aluminium, iron or copper or composition thereof, followed by addition of fly-ash upstream of primary clarifier and downstream of salt feed to increase the dry solids content of the influent raw wastewater by 10-50%, and then finally adding water soluble, surface active, branched polyether amine compound having specific gravity of 1.08, to the raw wastewater, upstream of the primary clarifier and downstream of the feed point of fly ash, in an amount sufficient to increase the settling rate of sludge forming material, and clarify the effluent water at the top of primary clarifier.
In the above process, the clarification is mainly achieved by sequential addition of chemicals, polymers and fly ash rather than the gravity clarification occurring in the clarifier itself. External addition of solids in the raw wastewater not only increases the size of clarifiers to accommodate increased sludge blanket, but also requires large sludge dewatering facilities. Further, the management of solids handling and disposal becomes tedious and means additional recurring cost due to increase in quantity of sludge generated, whereas the clarifier described in the present invention, can be used for primary clarification of raw wastewater without addition of fly ash.
Reference may be made to U.S. Pat. No. 5,306,422 (1994), which discloses a compact clarifier system for municipal wastewater treatment. The clarifier system is in general based on the principles of dissolved air floatation (DAF), wherein compressed air is released at the bottom of the tank in the form of microscopic bubbles that attach to the suspended solids, the bubbles carry the solids to the surface, where they form floating sludge. The method disclosed in the above-referred patent is energy intensive and requires highly skilled supervision as compared to method described in the present invention. The solids-liquid separation described in the present invention is based on the principle of gravitation, wherein solids and liquid are separated under quiescent condition, due to difference in specific gravity.
Reference may be made to U.S. Pat. No. 5,961,826 (1999) wherein a biological wastewater treatment system having a sedimentation tank vertically combined with an aeration tank has been disclosed. The inventors claim a sedimentation tank vertically combined with an upside aeration tank and connected with the said aeration tank through a through-hole, the sedimentation tank having a clarified water outlet for discharge of clarified water from the sedimentation tank. This system is only suitable for small-scale treatment plants, since such an installation is not feasible for large-scale wastewater treatment systems. Further, the treatment system described in the U.S. Pat. No. 5,961,826 does not claim any improvement in solids-liquid separation described in the present invention.
Reference may be made to U.S. Pat. No. 6,099,743 (2000) wherein a method and basin for sedimentation of solids in wastewater treatment has been disclosed. The method of removing solids from wastewater by flocculation comprises of forcing a flow of sludge containing wastewater to abruptly change direction a number of times in succession to form whirls that create an optimum velocity for formation of sludge flocs. A method and basin for sedimentation is suitable for a designed flow rate in order to obtain optimum velocity to promote flocculation; any change in the incoming flow rate will change the optimum velocity which will affect flocculation as described in the above referred invention, hence requires highly skilled supervision, whereas in the improved clarifier, the resistance to flow is offered gradually by solids itself as described in the present invention.